Exponential dilution flask

ABSTRACT

An exponential dilution flask for the calibration of gas detectors or for use as a dynamic gas blender or an efficient flow-through reactor for gas phase reactions. A nearly ideal first order exponential decay characteristic for the flask is obtained by introducing sample and carrier gases to a spherical shell and providing means in the shell for completely mixing incoming carrier fluid with the contents of the flask within the response time of a gas detector being calibrated. The fluid is moved by two fans of equal but opposite pitch rotating on a common shaft through oppositely facing corresponding holes in a pair of plates forming a mixing region therebetween. The fluid is thus divided by the holes into a plurality of separate streams which collide in the mixing region. A small portion of the combined fluid exits from the shell through a narrow outlet tube and the remainder is recycled for further division and combination.

United States Patent Ritter Oct. 14, 1975 EXPONENTIAL DILUTION FLASK[75] Inventor: Joseph J. Ritter, Mount Airy, Md. ABSTRACT 73 Assignee;The United States f America as An exponential dilution flask for thecalibration of gas represented by the Secretary of Commerce, Washington,DC.

Filed:

Appl. No.: 495,754

Aug. 8, 1974 FOREIGN PATENTS OR APPLICATIONS Primary ExaminerS. ClementSwisher Attorney, Agent, or FirmDavid Robbins; Alvin Englert Japan 73/ldetectors or for use as a dynamic gas blender or an efficientflow-through reactor for gas phase reactions. A nearly ideal first orderexponential decay characteristic for the flask is obtained byintroducing sample and carrier gases to a spherical shell and providingmeans in the shell for completely mixing incoming carrier fluid with thecontents of the flask within the response time of a gas detector beingcalibrated. The fluid is moved by two fans of equal but opposite pitchrotating on a common shaft through oppositely facing corresponding holesin a pair of plates forming a mixing region therebetween. The fluid isthus divided by the holes into a plurality of separate streams whichcollide in the mixing region. A small portion of the combined fluidexits from the shell through a narrow outlet tube and the remainder isrecycled for further division and combination.

15 Claims, 3 Drawing Figures 5 I7 7/ 4/ l0 l l zs 4 64 "I US. Patent vOct. 14, 1975 3,911,723

EXPONENTIAL DILUTION FLASK The present invention relates to an improvedexponential dilution flask which facilitates more accurate calibrationof gas detectors such as in air pollution monitoring instruments andwhich can also be used as a dynamic gas blender and as an efficientflow-through reactor for gas phase reactions.

It has been known in the prior art to utilize exponential dilutiontechniques for the calibration of gas detectors. However, due to thedeficiencies of prior art exponential dilution flasks, the calibrationsperformed in the prior art have generally not been reliable.

The calibration of a gas detector with an exponential dilution flask isbased on the theoretical expectation that the concentration of a sampleinjected into an exponential dilution flask through which a carrier gasis flowing will decay exponentially with time according to theexpression c e where c is the initial concentration and k is a constantwhich can be computed from the known system parameters. However, it hasbeen observed that the dilution processes in prior art flasks do notfollow an ideal first order exponential decay process according to theequation and that such flasks further exhibit poor slope reproducibilityon a day-today basis which renders them unsuitable for providingreliable calibrations.

The exponential dilution flasks of the prior art are basicallyspherical, cylindrical or bell-shaped vessels containing either flatsolid paddles or unshielded fans. It is suggested that the observedunreliability of these devices is due to inefficient blending of thesample and carrier gases which comprise the flask contents.

According to the invention it has been found that an exponentialdilution flask will follow the desired first order exponential decayequation only if the sample is randomly distributed throughout the flaskat all times. Since exponential dilution is a continuous dynamic processinvolving a steady influx of pure carrier gas and corresponding effluxof mixture of sample and carrier, the blending of incoming gas with theflask contents should ideally be instantaneous. In a practical system ittakes a finite time to move gas from one place to another to achievemixing but since all detector systems which will be measuring the effluxhave a finite response time (typically 0.1 to 0.5 seconds), it has beenfound that ideal behavior may be achieved to within a firstapproximation if the mixing is essentially complete within the responsetime of the detection and display system. According to the inventionthen a novel structure for an exponential dilution flask which meets theabove criteria has been provided.

It is thus an object of the invention to provide an improved exponentialdilution flask which is useful in the calibration of gas detectors suchas concentration detectors and detectors used in air pollutionmonitoring instruments and in determining the linearity of suchdetectors.

It is a further object of the invention to provide an improvedexponential dilution flask which more nearly follows an ideal firstorder exponential decay process.

It is still a further object of the invention to provide an exponentialdilution flask which achieves reproducible performance.

It is still a further object of the invention to provide an exponentialdilution flask in which the mixing of the incoming increment of fluid isessentially complete within the response time of a typical detection anddisplay system.

It is still a further object of the invention to provide a structurewhich may be used as a dynamic gas blender and as an efficientflow-through reactor for gas phase reactions.

According to the invention, an exponential dilution flask for mixing asample and carrier gas is provided which is comprised of a sphericalvessel which houses two ducted fans of equal but opposite pitch whichrotate with a common shaft. Gas is admitted into the vessel through aninlet tube and the fans move the gas at a relatively high volume flowthrough corresponding holes in opposing perforated plates. The gas isthus divided into laminar streams which collide head-on and blend. Theresultant mixture is then rapidly recirculated, redivided, blendedfurther, and a portion of the fluid exits through one of two portions ofa C-shaped exit tube provided in the vessel. The probability of a shortcircuit developing between the inlet and outlet is minimized since gasentering the device must pass through four mixing regions before havingaccess to the exit tube.

A better appreciation of the novel structure of the flask may be had byreferring to the figures which will be discussed in detail below butfirst, so that the advantages of the invention may be better appreciatedgas detector calibration techniques in general utilizing exponentialdilution will be discussed.

The basic calibration technique involves coupling the output of anexponential dilution flask to a gas detector (e.g., a flame ionizationdetector). The object of the calibration is to correlate the gasdetector signal outputs as displayed on a display device with actualconcentration values. Since in the ideal case the concentration of thegas exiting from the flask follows the formula c c,,e" where c is theinitial concentration, k is a constant known for the parameters of thesystem, and t is time, theoretically the concentration is known for eachinstant of time following t and this information may be used tocalibrate the detector. The advantage of this method as opposed toinputting several discrete samples of different known concentrations tothe detector is that it rapidly provides a continuous concentrationspectrum over several orders of magnitude.

Basically an exponential dilution flask is a gas mixing device of knownvolume Vo. A suitable carrier gas is purged through the flask at a knownconstant rate r. A premeasured quantity of pure sample N (moles) towhich the detector will respond is introduced into the exponentialdilution flask (EDF) at some time t If the sample is prefectly andinstantaneously mixed with the carrier gas at all times after itsintroduction then the instantaneous rate at which the sample leaves theflask (-dN/dt) is proportional to the amount of sample within the flaskat that instant. Thus dN/dt -N or a'N/dt kN where k is a constant. Theseequations are the basic mathematical expressions which describe a simplefirst order decay process and the concentration 0 of the sample at anytime t after t becomes 0 c e In the ideal case the initial concentrationof the sample c =N,,/Vg where V3 is volume of gas within the exponentialdilution flask (EDF) under actual operating conditions, k r/Vg and thesample concentration 0 at any time t after t can be calculated.

If the gas detector to which the EDF is coupled responds in a linearfashion it provides a signal (usually a voltage) S Ic where I constant.The area A under the curve generated by the detector is f3; Sdt. Bysubstitution of the above signal-concentration relationship andintegration between appropriate limits the constant I is found to be I=Ar/N Moreover, the slope k of the signal curve should be identical tothat calculated for the concentration curve, i.e. k, r/Vg. Fornon-linear detectors the quotient of k, and k can be used as a monitorof detector linearity. The linearity factor n is given by k,//

Thus for calibration purposes exponential dilution offers a rapid meansof providing a continuous concentration spectrum over several orders ofmagnitude, a signal-concentration relationship, and an assessment ofdetector linearity.

Of course, the above theory presupposes that the flask provides dilutionof the sample according to the ideal first order exponential decayequation and that the performance of the flask is reproducible, and itis in these areas where the exponential dilution flasks of the prior arthave fallen short. On the other hand, the structure described of thepresent invention in detail below provides a dilution flask whichpermits the desired theoretical results to be more nearly realized.

The invention will be better appreciated by referring to the Figures inwhich:

FIG. 1 is a schematic representation of an exponential dilution flaskaccording to the invention.

FIG. 2 is a cross-sectional view taken at A-A of FIG. 1.

FIG. 3 is a perspective view of elements 20,22 of FIG. 1.

Referring to FIG. 1, it is seen that the flask is spherical in shape.The sphere was chosen as the basic flask shape because it has no cornersand offers the smallest surface area for a given volume of all theregular geometric shapes thus maximizing the effective mixing region.

In FIG. 1 the sphere is formed partly of glass shell 6 and partly of theinside surface 62 of metallic member 7 which may be brass. Member 7 mayhave a circular cross-section in the plane perpendicular to the plane ofthe paper and parallel to wall 64. The inside surface 61,62 of themember is spherical in shape and has a ledge 63 into which the glassshell 6 is pressure fitted and sealed by O ring 8. A simple clampingframe (not shown) is used to retain the glass and metal sections.

Drive shaft 9 extends through longitudinally extending cylindricalopening 65 in member 7 and into the interior of the sphere. The shaft issupported in carbon filled PTFE bushing 10 and a simple rotary face seal11 to contain the gases within the sphere is comprised of the inner face66 of the bushing and a face of stainless steel ring 12 which isstatically sealed and fixed to the drive shaft with an O ring 13. Theinside diameter of stainless steel ring 12 is bored slightly larger thanthe drive shaft diameter to permit the ring face to move parallel to thebushing face. The seal is kept under tension by any mechanicalarrangement known to those skilled in the art for exerting a pullingforce on shaft 9 which force will cause ring 51 and member 14 to pushring 12 against face 66. One such exemplary arrangement is shown in FIG.1 wherein spring 53 pushes thrust washer 27 against a face of bushing 52at one end and against a face of ring 54 at the other end. Bushing 52 isarranged to be immovable (not shown) and ring 54 is attached to theshaft 9 so that the effect of the spring is to continuously pull theshaft 9 outwardly from the sphere. The type of seal used besides beingsimple has the advantage of improving rather than deteriorating withuse.

A conical aluminum member 14 is locked to the drive shaft by means of asmall set screw, not shown. Also mounted on the shaft and locked theretowith set screws are cylindrical spacer 18 and conical member 19. Twofans 15 and 16 of equal but opposite pitch are made to rotate with thedrive shaft by being frictionally locked between members 14 and 18 and18 and 19 respectively by 0 rings 17 and 41. In the preferred embodimentthe fans may be made of aluminum, the pitch of the fan blades is 30 andthe diameter thereof is 3.8 centimeters.

Fan blade 15 is surrounded by a stationary brass shroud 20 and fan blade16 is surrounded by a stationary brass shroud 21. Each shroud has 16equally spaced peripheral holes, and 43 respectively, drilled therein at45 to the drive axis. Shroud 20 is fitted with a facing place 22 whichin the preferred embodiment may be l/16 of an inch thick brass andshroud 21 is fitted with a similar facing place 23. Each facing platecontains four concentric groups of holes, and groups 31, 32, 33 and 34of plate 22 are arranged to be situated exactly opposite from thecorresponding groups of plate 23. A perspective view of shroud/facingplate unit 20,22 is shown in FIG. 3 and as illustrated, in the preferredembodiment each concentric group of holes has 25 holes. As will bedescribed below, the perforated plates serve both to remove therotational components induced in the gas by the fans and to divide thegas into separate streams.

Shroud assemblies 20,22 and 21,23 may be mounted in the flask by anyconvenient mechanical means so long as no part of the assembly touchesspacer 18, the annulus 71 preferably being about 1/32 inch in width. Onemounting mode found to be advantageous is to connect the two shroudassemblies together by three metal supports soldered to members 22 and23 at their outer peripheries at positions 120 displaced from each otherand extending parallel to shaft 9. Member 20 is provided with threemetal feet supports which may be straight rods also soldered to edge 72at positions 120 displaced from each other, extending parallel to shaft9 and resting at their other ends on portion 62 of metal member 7 justbelow 0 ring 8. Three phosphor bronze spring clips are attached to edge73 of member 21 at positions 120 displaced from each other. The springclips also extend parallel to shaft 9 and the outside ends springagainst the inside of spherical glass shell 6 thereby pushing bothshrouds to the left in the drawing and forcing the feet connected toedge 72 against portion 62 of metallic member 7.

Conical member 14 is enclosed by stationary brass shroud 24 whichcontains 16 equally spaced peripheral holes drilled at to the driveshaft axis. The input end of inlet passageway 25 is drilled in metallicmember 7 as shown in FIG. 1, and the output portion extends through tube39 which is soldered or welded to member 7, shroud 24 being soldered totube 39. The output of passageway 25 is arranged to dischargetangentially into the annular space between shroud 24 and conical member14 as shown more clearly in the crosssectional view of FIG. 2.

The blended gas escapes from the sphere by entering C-shaped tube 26 ateither end thereof. C-shaped tube 26 terminates at its middle in flaskoutlet tube 40 which provides an outlet for the gas. Tube 26 isrelatively narrow and in the preferred embodiment has an inside diameterof 0.102 centimeters.

In the operation of the device, shaft 9 is driven at ap-' proximately5500 rpm by an air turbine and the sample and carrier gases are admittedthrough inlet tube 25. The incoming gas enters tangentially to thespiral flow pattern generated in the fluid between spinner member 14 andits stationary shroud 24 as shown in FIG. 2 to the region denoted as 1in FIG. 1. Upon reaching the end of the shroud, the now pre-mixed inletgas is divided into sixteen separate streams by the sixteen peripheralholes 42 and is discharged into region 2 where the first fan is located.In so doing, the streams must transverse the fluid being drawn into thefan and further blending is effected. Conical spinners l4 and 19 on theintake sides of fans 15 and 16 respectively assist flow into the workingregion of the blades and reduce fluid stagnation near the shaft.

The mixture is moved by the motion of fan 15 to the area at the far endof shroud 20 which has 16 peripheral holes therein and to face plate 22which has 100 holes therein. At the same time, fluid mixture is beingdrawn past conical spinner 19 by fan blade 16 and is moved to face plate23 and the far end of shroud 21. Thus the mixture, after it has movedpast each of the fan blades, is subdivided into 1 l6 streams, 100 ofwhich collide head-on at velocities of between 160 to 200 centimetersper second in region 3 shown in FIG. 1. The resulting blend is thenforced by pressure differential and the radial pumping induced by therotating shaft to move outwardly towards the walls of the sphere toregion 4. In so doing, it must traverse a cross-fire of 32 streamsemanating from the peripheral holes and 43 of shrouds 20 and 21respectively. The angle of holes 30 and 43 is arranged so that thestreams do not collide, but rather pass each other inducing vortexformation and mixing at the point of passing. Most of the fluid thuscompounded is recycled while a small portion finds exit through one ofthe entrance ends of C- shaped tube 26 to the outlet 40.

The probability of a short circuit developing between inlet and outletis very low, since any gas entering the device must traverse all fourmixing regimes before having access to the exit tubes. The effects offluid stagnation near the walls and other stationary portions of thedevices are minimized by providing polished surfaces and gentle curveswherever possible. At a shaft speed of 5500 rpm the estimatedrecirculation rate within a flask having a volume of 150 ml is 20 timesper second. In experiments performed with the structure of the inventionthe sample propane was investigated using a carrier gas of air ornitrogen.

Two calibration methods utilizing the exponential dilution flask of theinvention which have been found to be particularly suitable will now bedescribed. In the first method, an approximate quantity of pure samplegas is introduced into the flask which is purged with carrier gas andthe detector output as a function of time is recorded. This is used tocompute the detector linearity factor n which is the ratio of theobserved slope of the log of the recorded signal versus time curve tothe theoretical value discussed above computed from the volume of gas inthe EDF, Vg, and gas flow rate. All gas volumes and flow rates arecorrected to STP. A single high-level response So within theconcentration region covered by the dilution curve is established with acertified span gas of known concentration. All responses S below thespan gas level can now be expressed in terms of calibrant concentration0 by use of the equation This technique does not require the use of aprecalibrated span gas mixture as does the above method. It furthershould be noted that apart from calibrating the detector it is sometimesnecessary only to determine the linearity factor of the detector and theexponential dilution flask of the invention may be used for thispurpose.

While I have disclosed and described the preferred embodiments of myinvention, 1 wish it understood that I do not intend to be restrictedsolely thereto, but that I do intend to include all embodiments thereofwhich would be apparent to one skilled in the art and which come withinthe spirit and scope of my invention.

1 claim:

1. An exponential dilution flask comprising a substantiallysphericalshell, means for introducing sample and carrier fluid to saidshell, means in said shell for dividing said introduced fluid into aplurality of separate fluid streams and for combining said fluidstreams, outlet means for outputting a small portion of said combinedfluid from said shell, and means for recylcling the remainder of saidcombined fluid to said means for dividing and combining for re-divisionand recombination.

2. The exponential dilution flask of claim 1 wherein said means fordividing and combining comprises first and second members located insaid shell downstream of said introduction means and forming a mixingregion therebetween, each of said members having a plurality ofcorresponding holes therein which face each other across said mixingregion, and means for simultaneously moving fluid through the holes ineach of said members towards said mixing region whereby the fluid isdivided by said holes in each of said members and combined in saidmixing region.

3. The exponential dilution flask of claim 2 wherein said means forsimultaneously moving said fluid is comprised of first and second fanmeans of equal but opposite pitch located outside of the sides of saidfirst and second members respectively which face away from said mixingregion, and means for simultaneously rotating said first and second fanmeans.

4. The exponential dilution flask of claim 3 wherein said means forsimultaneously rotating comprises a shaft which passes through thecenters of said fan means.

5. The exponential dilution flask of claim 4 wherein said shaft hasfirst and second conical spinners mounted thereon adjacent said firstand second fan means respectively to assist fluid flow into said fanmeans.

6. The exponential dilution flask of claim 5 wherein said first andsecond members each comprise a flat plate having a plurality of rings ofconcentric holes therein, said plates being perpendicular to said shaft.

7. The exponential dilution flask of claim 6 wherein said means fordividing further includes first and second cylindrical members which areconcentric with said shaft and surround said first and second fan meansrespectively, said cylindrical members having a plurality of holes inthe periphery thereof cut at an acute angle to the direction of saidshaft, said first and second cylindrical members being joined at theends thereof to said first and second plane members respectively.

8. The exponential dilution flask of claim 7 wherein said outlet meanscomprises a narrow tube which leads to the exterior of said shell.

9. The exponential dilution flask of claim 8 wherein said tube comprisesa C-shaped tube each open end of which admits fluid, said tube providinga passageway to the exterior of said shell at its mid-section.

10. The exponential dilution flask of claim 8 wherein said introductionmeans for said fluid comprises an inlet opening which directs fluid intosaid spherical shell at said first spinner in a direction tangential tothe surface of said first spinner.

ll. The exponential dilution flask of claim 10 wherein said firstspinner is surrounded by a cylindrical member concentric with and spacedfrom said spinner, said cylindrical member have a plurality of holes inthe periphery thereof cut at an acute angle to the direction of saidshaft.

12. The exponential dilution flask of claim 11 wherein said sphericalshell is sealed by a seal comprised of a bushing in which said shaftrotates and an axially movable ring mounted on said shaft, means beingprovided for pulling said shaft outwardly from said sphere so that saidfirst spinner pushes said ring against said bushing to form said seal.

13. The exponential dilution flask of claim 12 wherein said means forpulling includes a spring means and a thrust washer.

14. The exponential dilution flask of claim 4 wherein said sphericalshell is comprised partly of a glass shell and partly of the interior ofa metallic frame member said metallic member having a longitudinal holetherein through which said shaft passes.

15. A method of mixing a sample gas and a carrier gas comprising,

providing a substantially spherical shell,

introducing said sample gas and said carrier gas to said shell,

dividing said introduced gas into a plurality of gas streams,

causing some of said streams to collide head on with others of saidstreams to effect mixing,

outputting a small portion of said mixed gas from said shell,

recycling the remainder of said mixed gas for redivision andre-combination.

1. An exponential dilution flask comprising a substantially sphericalshell, means for introducing sample and carrier fluid to said shell,means in said shell for dividing said introduced fluid into a pluralityof separate fluid streams and for combining said fluid streams, outletmeans for outputting a small portion of said combined fluid from saidshell, and means for recylcling the remainder of said combined fluid tosaid means for dividing and combining for re-division andre-combination.
 2. The exponential dilution flask of claim 1 whereinsaid means for dividing and combining comprises first and second memberslocated in said shell downstream of said introduction means and forminga mixing region therebetween, each of said members having a plurality ofcorresponding holes therein which face each other across said mixingregion, and means for simultaneously moving fluid through the holes ineach of said members towards said mixing region whereby the fluid isdivided by said holes in each of said members and combined in saidmixing region.
 3. The exponential dilution flask of claim 2 wherein saidmeans for simultaneously moving said fluid is comprised of first andsecond fan means of equal but opposite pitch located outside of thesides of said first and second members respectively which face away fromsaid mixing region, and means for simultaneously rotating said first andsecond fan means.
 4. The exponential dilution flask of claim 3 whereinsaid means for simultaneously rotating comprises a shaft which passesthrough the centers of said fan means.
 5. The exponential dilution flaskof claim 4 wherein said shaft has first and second conical spinnersmounted thereon adjacent said first and second fan means respectively toassist fluid flow into said fan means.
 6. The exponential dilution flaskof claim 5 wherein said first and second members each comprise a flatplate having a plurality of rings of concentric holes therein, saidplates being perpendicular to said shaft.
 7. The exponential dilutionflask of claim 6 wherein said means for dividing further includes firstand second cylindrical members which are concentric with said shaft andsurround said first and second fan means respectively, said cylindricalmembers having a plurality of holes in the periphery thereof cut at anacute angle to the direction of said shaft, said first and secondcylindrical members being joined at the ends thereof to said first andsecond plane members respectively.
 8. The exponential dilution flask ofclaim 7 wherein said outlet means comprises a narrow tube which leads tothe exterior of said shell.
 9. The exponential dilution flask of claim 8wherein said tube comprises a C-shaped tube each open end of whichadmits fluid, said tube providing a passageway to the exterior of saidshell at its mid-section.
 10. The exponential dilution flask of claim 8wHerein said introduction means for said fluid comprises an inletopening which directs fluid into said spherical shell at said firstspinner in a direction tangential to the surface of said first spinner.11. The exponential dilution flask of claim 10 wherein said firstspinner is surrounded by a cylindrical member concentric with and spacedfrom said spinner, said cylindrical member have a plurality of holes inthe periphery thereof cut at an acute angle to the direction of saidshaft.
 12. The exponential dilution flask of claim 11 wherein saidspherical shell is sealed by a seal comprised of a bushing in which saidshaft rotates and an axially movable ring mounted on said shaft, meansbeing provided for pulling said shaft outwardly from said sphere so thatsaid first spinner pushes said ring against said bushing to form saidseal.
 13. The exponential dilution flask of claim 12 wherein said meansfor pulling includes a spring means and a thrust washer.
 14. Theexponential dilution flask of claim 4 wherein said spherical shell iscomprised partly of a glass shell and partly of the interior of ametallic frame member said metallic member having a longitudinal holetherein through which said shaft passes.
 15. A method of mixing a samplegas and a carrier gas comprising, providing a substantially sphericalshell, introducing said sample gas and said carrier gas to said shell,dividing said introduced gas into a plurality of gas streams, causingsome of said streams to collide head on with others of said streams toeffect mixing, outputting a small portion of said mixed gas from saidshell, recycling the remainder of said mixed gas for re-division andre-combination.